1. Field of the Invention
The invention relates to a cooling system adapted to cool an internal combustion engine by circulating a coolant and effecting heat exchange between the coolant and the internal combustion engine.
2. Description of Related Art
A known cooling system for a water-cooled engine installed on a motor vehicle, or the like, includes a radiator provided in a coolant circulation path of the engine for cooling a coolant or cooling water, and a flow control valve that controls the flow rate of the coolant that passes through the radiator. In this type of cooling system, the temperature of the engine coolant changes in accordance with the flow rate of the coolant that is controlled through control of the opening of the flow control valve.
One example of the control of the opening of the flow control valve is disclosed in, for example, Japanese Laid-open Patent Publication No. 5-179948. Under the valve opening control disclosed, a target coolant temperature is set based on the engine load and the engine speed. Then, the opening of the flow control valve is controlled in a feedback fashion so that the actual engine coolant temperature is made equal to the set target coolant temperature. With this control, the flow rate of the coolant passing through the radiator is controlled, and the engine coolant temperature approaches and becomes substantially equal to the target coolant temperature.
With the known technology as described above, the temperature of the coolant is controlled depending upon the load state or condition of the engine. When the engine is required to generate a high level of driving power, therefore, the coolant temperature is lowered so as to increase the cooling efficiency of cylinders of the engine. When the engine is required to operate with a low fuel consumption (i.e., at a high fuel efficiency), on the other hand, the coolant temperature is elevated so as to increase the combustion efficiency in the cylinders. In this manner, the coolant temperature is controlled so as to achieve sufficiently high levels in opposite performances or characteristics, i.e., high power (output performance) and low fuel consumption.
In the cooling system disclosed in the above-identified publication, control of the opening of the flow control valve is performed based only upon a difference between the actual coolant temperature and the target coolant temperature. Therefore, the cooling system suffers from poor response when controlling the coolant temperature to the target coolant temperature. In particular, when a quantity of heat equivalent to a cooling loss of the engine changes with a change in the operating state of the engine, the coolant temperature cannot be controlled to the target coolant temperature with good response. Here, the coolant loss is a quantity of heat removed from the engine and radiated or absorbed into the coolant in the process in which the coolant passes through the engine. If the coolant loss changes as described above, a power loss takes place which is detrimental to improvements in the fuel efficiency and the output performance. A similar problem may be encountered in a cooling system in which the flow rate of coolant passing through a radiator is controlled by an electric water pump, in place of the flow control valve.